# europepmc.citations

**stable** **Advanced**

Europe PMC articles citing a PMID.

`GET /v1/europepmc/citations`

## What you get

A list of Europepmc results in Onekey's unified shape, in `data` (an array). An empty array is a normal, successful answer and is charged like any other; a source that reports "nothing found" as an error returns a free `404 not_found` instead.

## What it costs

- **Advanced · 5 credits** for a fresh fetch, charged only when it succeeds.
- **Free when served from cache (0.2 credit for a cached response over 100 KB).**
- **Free** if the request is invalid (never charged) or the source fails or reports not found (refunded).

## Parameters

| Name | Type | Required | Default | What it is | Constraints |
| --- | --- | --- | --- | --- | --- |
| `pmid` | string | yes | (required) | PubMed id (PMID) | max 9 chars; must match `[0-9]{1,9}`; trimmed |

## Request

```bash
curl "https://api.onekey.dev/v1/europepmc/citations?pmid=33087721" \
  -H "x-api-key: YOUR_API_KEY"
```

```python
import requests

res = requests.get(
    "https://api.onekey.dev/v1/europepmc/citations",
    params={"pmid": "33087721"},
    headers={"x-api-key": "YOUR_API_KEY"},
)
body = res.json()

if body["success"]:
    print(body["data"])
else:
    print(body["error"]["code"], body["error"]["message"])
```

```javascript
const params = new URLSearchParams({ pmid: "33087721" });
const res = await fetch(`https://api.onekey.dev/v1/europepmc/citations?${params}`, {
  headers: { "x-api-key": "YOUR_API_KEY" },
});
const body = await res.json();

if (body.success) console.log(body.data);
else console.error(body.error.code, body.error.message);
```

## Recorded response

A real Europepmc response, recorded and returned in Onekey's shape. Fields the platform does not provide are `null`.

```json
{
  "success": true,
  "data": [
    {
      "content": {
        "id": "42647127",
        "title": "Engineering antigenic breadth against SARS-CoV-2 by pairing divergent RBDs within a single mRNA immunogen.",
        "year": 2026
      },
      "author": {
        "displayName": "Montgomerie I, McKenzie RE, Palmer OR, Mason NC, Kuang J, Pankhurst TE, Draper SL, von Daake S, Eccles DA, Bird TW, Martin AL, Green I, White L, Robinson Z, Marshall A, Minnell JJ, Small SJ, Hermans IF, Painter GF, Ussher JE, Quiñones-Mateu ME, Patrick WM, Comoletti D, Connor LM."
      },
      "ext": {
        "source": "MED"
      }
    },
    {
      "content": {
        "id": "PPR1283099",
        "title": "A human monoclonal antibody from hybrid immunity exhibits exceptional neutralization breadth against antigenically divergent contemporary Omicron JN.1 lineage variants",
        "year": 2026
      },
      "author": {
        "displayName": "Rathour D, Shrivas S, Kumar N, Prasad C, Singh J, Singh G, Surendranath A, Rathi S, Verma S, Chauhan S, Singh B, Sutar J, Chattopadhyay S, Batra G, Sonar S, Eckerle I, Subbissi L, Mohan A, Asthana S, Deshpande S, Bhattacharya J."
      },
      "ext": {
        "source": "PPR"
      }
    }
  ],
  "metadata": {
    "platform": "europepmc",
    "endpoint": "citations",
    "cached": false,
    "stale": false,
    "age_s": 0,
    "fetched_at": "2026-01-01T12:00:00+00:00",
    "credits_charged": 5,
    "request_id": "7f3a9c2e5b1d4c8fa2e6d0b9c4f18e37"
  },
  "error": null
}
```

## Field mapping

How this endpoint's source fields become Onekey's [unified schema](https://onekey.dev/docs/schema.md). Fields the source lacks come back as `null`.

| Source path | Onekey field | Transform |
| --- | --- | --- |
| `id` | `content.id` | - |
| `title` | `content.title` | - |
| `authorString` | `author.displayName` | - |
| `pubYear` | `content.year` | `toNumber` |
| `source` | `ext.source` | - |

## Caching and freshness

- Fresh for about 1 hour. Repeat calls in that window are served from cache and are free.
- Between 1 hour and 1 day the cached copy is still served instantly and marked `stale: true` while Onekey refreshes it in the background.
- If the source is down when a refresh is needed, the last copy can be served for up to 3 days more (marked stale).
- A "not found" is remembered for 5 minutes and is free.
- The cache is shared across all customers, which is what keeps repeat calls cheap.

## What can go wrong

| Status | Code | What to do |
| --- | --- | --- |
| 401 | `unauthorized` | Check the `x-api-key` header. |
| 402 | `insufficient_credits` | [Add credits](https://onekey.dev/pricing); your balance must cover the price. |
| 404 | `not_found` | Unknown endpoint path. |
| 422 | `invalid_params` | Fix the parameters using the table above. Free. |
| 429 | `rate_limited` | Wait `Retry-After` seconds, then retry. |
| 502 | `upstream_error` | The source failed. Refunded; retry with backoff. |

See [Errors](https://onekey.dev/docs/errors.md) for the full list and retry guidance.

Status: stable (verified against the live source).